Many systems have sensitive portions such as volatile memories, clocks or the like which continuously need to receive a supply voltage .vertline.v.sub.S .vertline.. During normal operation, all portions including the sensitive portions receive supply voltages (e.g., .vertline.v.sub.S .vertline.) from main batteries (e.g., DC), from the mains (e.g., AC) or from other sources. During stand-by operation, only the sensitive portions receive .vertline.v.sub.S .vertline. from a back-up battery. At all times, the supply voltage .vertline.v.sub.S .vertline. at the sensitive portions should have a minimum value .vertline.v.sub.min .vertline., that is: EQU .vertline.v.sub.S .vertline..gtoreq..vertline.v.sub.min .vertline.(1)
Voltages are given here as absolute values (symbols .vertline..vertline.) wherein the polarity (plus or minus) is not important.
For example, systems can go from active operation to stand-by operation by an intentional action of a user (e.g., switching off or replacing the battery), by mistake (operating error), or due to a technical failure. Conveniently, the back-up batteries are often coupled in parallel to outputs of voltage regulators. To save costs, the back-up batteries should be discharged only in the stand-by operation, while parasitic discharging during active operation should be minimized. Modern systems require low supply voltages .vertline.v.sub.S .vertline., such as, for example, 3.3 volts, 1.8 volts, or with even lower values.
For the application of power supply circuits and for prior art designs, the following references are useful: Wolbert, B.:"Designing with Low-Dropout Voltage Regulators", published by Micrel Semiconductor, 1849 Fortune Drive, San Jose, Calif. 95131 [1]; U.S. Pat. No. 5,375,246 to Kimura [2]; U.S. Pat. No. 4,422,163 to Oldenkamp [3]; and U.S. Pat. No. 4,122,359 to Breikss [4].
FIG. 1 is a simplified block diagram of prior art supply system 100. System 100 comprises memory circuit 110, back-up battery 120, voltage sensor 130, regulator 150, and diode 190. Regulator 150 and voltage sensor 130 form supply circuit 160 (dashed frame). Regulator 150 receives a voltage .vertline.v.sub.1 .vertline. between input terminal 162 and reference line 101. Voltages having an integer subscript (e.g., .vertline.v.sub.1 .vertline.) are referred to reference line 101. Regulator 150 provides a voltage .vertline.v.sub.2 .vertline. on line 102. Preferably, the magnitudes of .vertline.v.sub.1 .vertline. and .vertline.v.sub.2 .vertline. are related as: EQU .vertline.v.sub.1 .vertline.&gt;.vertline.v.sub.2 .vertline. (2)
Voltage sensor 130 is coupled between lines 102 and 101 and receives voltage .vertline.v.sub.2 .vertline.. Voltage sensor 130 feeds back a measurement signal 132 on line 131 to regulator 150. Voltage sensors are well known in the art. For example, voltage sensor 130 can be implemented by two serially coupled resistors in a voltage divider arrangement.
Diode 190 is coupled to line 102 and to line 170. Back-up battery 120 and memory circuit 110 are parallel coupled between lines 170 and 101. When system 100 is active, regulator 150 receives voltage .vertline.v.sub.1 .vertline. and provides voltage .vertline.v.sub.2 .vertline.. Voltage .vertline.v.sub.2 .vertline. is preferably constant and smaller than voltage .vertline.v.sub.1 .vertline.. Diode 190 propagates .vertline.v.sub.2 .vertline. to .vertline.v.sub.3 .vertline.. Voltage .vertline.v.sub.3 .vertline. is the supply voltage .vertline.v.sub.S .vertline. for memory circuit 110. Optionally, voltage .vertline.v.sub.3 .vertline. can charge back-up battery 120.
When system 100 operates in the stand-by mode, regulator 150 does not provide .vertline.v.sub.2 .vertline.. Regulator 150 can optionally receive voltage .vertline.v.sub.1 .vertline., but this is not important. Back-up battery 120 provides voltage .vertline.v.sub.4 .vertline. which becomes the supply voltage .vertline.v.sub.S .vertline. for memory circuit 110. Now, back-up battery 120 is discharging. Voltage .vertline.v.sub.4 .vertline. of back-up battery 120 can be equal to, higher or lower than voltage .vertline.v.sub.3 .vertline.. In any case, during active operation and during stand-by operation, diode 190 prevents the backflow of a parasitic current generated by back-up battery 120 through voltage sensor 130. In other words, a uni-directional current path goes from regulator 150 to memory circuit 110.
However, diode 190 causes a voltage drop .vertline.v.sub.D .vertline. between voltages .vertline.v.sub.2 .vertline. and .vertline.v.sub.3 .vertline.: EQU .vertline.v.sub.D .vertline.=.vertline.v.sub.2 .vertline.-.vertline.v.sub.3 .vertline. (3)
Such voltage drop .vertline.v.sub.D .vertline. of about 0.3 to 0.6 volts is often not wanted. Voltage .vertline.v.sub.2 .vertline. is larger than supply voltage v.sub.S. This is especially inconvenient in a low supply voltage environment.
The present invention seeks to provide supply systems which mitigate or avoid these and other disadvantages and limitations of the prior art.